At a Glance

A factor VIII inhibitor is an antibody directed against factor VIII. The factor VIII inhibitor is quantitated in Bethesda units. One Bethesda unit of factor VIII inhibitor produces a residual factor VIII concentration of 50%. A 2nd Bethesda unit reduces the level to 25-50, and so forth, with each additional Bethesda unit. Thus, seven Bethesda units results in a factor VIII level of less than 1% of normal.

With rare exceptions, factor VIII inhibitors do not decrease the concentration of von Willebrand factor. Antibodies to factor VIII can appear in hemophilia A patients, and they can also appear spontaneously in nonhemophiliac patients. There are a variety of associated disorders or conditions associated with the development of spontaneous inhibitors to factor VIII. These include malignancy, drug reactions, SLE, rheumatoid arthritis and the peripartum state. In nonhemophiliac patients who spontaneously acquire inhibitors to factor VIII, nearly 75% are older than 50 years of age. There is no correlation between transfusion requirement and the development of a factor inhibitor.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

For a diagnosis of a factor VIII inhibitor, the partial thromboplastin time (PTT) must be increased, the factor VIII must be decreased, the PTT mixing study should show either partial or complete correction immediately after mixing and the correction should fade toward an elevated PTT after 30 minutes to 1 hour of incubation of the mixed plasma at 37°C. This is a classic pattern for the mixing study that is different from the pattern seen with a lupus anticoagulant or a simple factor deficiency. When a factor VIII inhibitor is present, it is essential to determine the number of Bethesda units, because treatment depends on this result.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

A lupus anticoagulant, if present, will usually not show a correction into the normal range at any time in the PTT mixing study, unlike a factor VIII inhibitor. The presence of a lupus anticoagulant should result in a positive lupus anticoagulant test, but a factor VIII inhibitor can produce a false-positive lupus anticoagulant test. Thus, the pattern of the mixing study is an important point of differentiation between a lupus anticoagulant and a factor VIII inhibitor. In addition, a factor VIII inhibitor shows a relatively selective decrease in factor VIII, whereas a lupus anticoagulant can produce low levels of factors VIII, IX, XI and/or XII.

What Lab Results Are Absolutely Confirmatory?

The presence of a Bethesda unit level of at least 1 unit per ML in the presence of a decreased factor VIII and in the absence of a lupus anticoagulant confirms the presence of a factor VIII inhibitor.

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